Electric circuit for color correction



July 15, 1941.

BLUE

7 CIRCUIT GREEN C/QCU/T' RED C/RCU/T v. c. HALL 2,249,522

ELECTRIC CIRCUIT FOR COLOR CORRECTION Filed Oct. 11, 1938 2 SheeTls-Sheet 2 F/y. a

I l l I I I L/NEAR w LINEAR LINEAR AMPLIFIER AMPLIFIER AMPLIFIER r LIGHT 1/40 15 626 31 3 6 3? 6 i 56 1v I r LINEAR f Iva/y Ll/YEA LINEAR jag 5 2i AMPL/F/ER v AMPL/F/ER AMPLIFIER AMPLIFIER 30\\ I 1 I i i 3/ az ,L/GHT VAL v5 L/NEAR NQIYL/IYEAR LINEAR AMPL lF/E AMPLIFIER AMPLIFIER z z 2 R /2,Q L31? /4/? INVENTOR ATTORNEYS Patented July 15, 1941 UNETED STATES ELECTRIC CHRCUIT FQR QOLGR CORRECTION Application October 11, 1938, Serial No. 234,422

11 Claims. .(Cl. 178--5.2)

This invention relates to electro-optical systems for the reproduction of multilolored originals. It relates particularly to a method and means for introducing color correction in such systems.

In a copending application, Serial Number 120,964, filed January 16, 1937, A. Murray and R. S. Morse describe the broad idea of introduding the color correction in electro-optical systems of this type. In a copending application filed Nov. 19, 1938, Serial Number 241,433, I have described the application of this broad idea to the problem of correcting errors which occur in the duplication of monopack color transparencies. applied to the specific idea of my cop-ending application as well as to the system broadly disclosed by Murray and Morse. It may also be applied to the transmission of colored pictures by wire or television.

It is an object of the present invention to give improved color correction andbetter control over the amount and type of color correction than hitherto available.

The type of electro-optical systems to which i the invention belongs usually include some scanning device and a beam-splitter together with color filters, for example, the three primary colors, for scanning the original in plurality of colors. Each of the colored scanning beams is directed to a photoelectric cell which together with its amplifying circuit and a light valve forms an electric channel carrying an electric energy in accordance with the intensity of the corresponding scanning beam. The analogous systems in television or sending pictures by wire are obvious from the following description and will not be separately considered. lhe light valve which may be of the ribbon type or a glow lamp controls the printing of a color separation negative corresponding tothe color falling on the photoelectric cell or any type of light controlling devices such as used in reproduction of pictures sent by television. According to the Murray and Morse application above mentioned,

in the channel. Secondly, the modification of The present invention may be the energy in one channel may be in accordance with a non-linear power function of the energy in one of the other channels.

More specifically, one or more of the channels include a non-linear amplifier having an output to input transmission function between the .25 and unit power, 1. e., between the fourth root and linear. By modulating the output of the photoelectric cell in another channel in accordance with the output of this non-linear amplifier, a degree of modulation proportional to the transmission function of the non-linear amplifier is obtained. On the other hand, the energy in a circuit having only linear amplifiers may be used to modify theoutput of this nonlinear amplifier and hence the degree of modification in this channel which contains the nonlinear amplifier will be proportional to a power between 4 and 1 of the energy in the other channel.

In the mathematical comparison of electro optical and photographic systems of color reproduction the following features are analogous. The reflectivity of the original to red, green and blue is the same, in both cases. The exposure of-the photographic negative and the exposure of the photoelectric cell are the same and are directly proportional to the original reflectivity. In the electrical case the signal is proportional to the exposure of the celland any number of linear amplifiers in the circuit merely changes the proportionality factor. In the photographic case, on the other hand, the negative density is not linearly proportional to the exposure but follows a characteristic curvepart of which involves a logarithmic relationship between density and exposure. The factor in this relationship over the purely logarithmic portion of this characteristic curve is called the contrast or gamma of the photographic emulsion.

In straight color reproduction this contrast for each color must be controlled and balanced against the other colors. Furthermore, in color correction of the masking type a high contrast negative is masked by a low contrast positive. In fact, in any type of color correction it, is desirable and necessary to be able to control the contrast and/or similar factors.

Thus, the present invention provides in electro optical systems a control similar to, but in many ways better than, the control of contrast in a photographic system.

The objects and advantages of the invention and the invention will be more clearly understood from the following description when read in connection with the accompanying drawings in which:

Fig. 1 shows an adjustable form of the invention.

Fig. 2 shows one form of electric circuit which can be used to provide the embodiment shown in Fig. 1.

Fig. 3 shows another embodiment of the invention involving various forms thereof.

In Fig. 1 the green channel includes a green filter IUG through which a corresponding scanning beam falls on a photoelectric cell l IG having a linear amplifying circuit I2G, a non-linear amplifier I3G, a second linear amplifier MG in series to operate a light valve [5G in any of the usual way. The non-linear amplifier [3G was introduced, according to the invention to permit correction for rate of change of contrast of the input energy. The blue channel similarly includes a filter [9B, photoelectric cell IIB, linear amplifiers I23 and MB, a non-linear amplifier I3B, and a light valve I5B. According to Murray and Morse the energy in the blue channel should be modified in accordance with the energy in the green channel. Applied to this Fig. 1, their system would be such that the output of the linear amplifier IZG through a suitable modifier such as shown at l6 would modify the output of the linear amplifier IZB. According to the present invention the energy in the blue channel is modified in accordance with some non-linear power function of the energy in the green channel. If, for example, the non-linear amplifier I3G has a square root transmission function whereby its output is proportional to the square root of its input, the output of the linear amplifier I2B will be modified through a modifier I8 in accordance with the square root of the energy in the amplifier IZG. Non-linear'amplifiers are well known and the type here described are those which employ variable resistances 29; these variable resistances are commonly designated varistors. They thus contain means for varying the output by some exponential factor which is substantially constant over a wide range of intensities. By suitable adjustment of the resistances in the modifiers I 6 and I8 the modification of the energy in the blue channel may be made proportional to any desired additive combination of the linear function and the square root function of the energy in the green channel.

The modification may be linear subtraction, e. g. by merely superimposin the potentials corresponding to the energies in opposite directions across a resistance. It may be division, e. g. by adjusting a resistance in the blue channel in accordance with the energy in the green channel. It may be multiplication as shown in Fig. 2, where the energy in the blue channel is multiplied by a constant minus the modifying energy which may for example be proportional to the square root of the energy in the green channel. The term modify is used to include all such modifications or embodiments.

If the non-linear amplifier [3B in the blue channel has a transmission factor for example according to the third root power, a modifier I! which introduces the output of the linear amplifier IZB into the linear amplifier MB will effectively modify the output of the blue channel in accordance with the third power of the energy in the green channel. A fourth modifier [9 would in the example taken permit modification of the energy in the blue channel in accordance with the three halves powerof the energy in the green channel. In all of these cases the modification is uniform over practically the whole range of intensities so that, over the major portion of the total latitude, the linearity of density relationships is maintained, at least approximately. Since density is logarithmic this maintenance of linearity with respect thereto is really a maintenance of the constant of non-linearity with respect to intensities themselves.

If it is desired to modify the energy in the blue channel in accordance with some non-linear power function of the energy in the green channel, but to have the light valve ISG react in direct proportion to the input from the photoelectric cell HG, the light valve I5G may be operated directly from the linear amplifier 12G and the output of the non-linear amplifier 13G to the linear amplifier IG would be used only for modification of the energy in the blue channel.

In Fig. 2 there is shown a detailed electric circuit corresponding to Fig. l. A portion of the output of the photoelectric cell HG is taken off through a con-denser 2E! and a rectifier 22 to modulate the energy in the blue circuit in the linear amplifier [23 before the non-linear amplifier 1313 and/or alternatively in the linear amplifier I413 after the non-linear amplifier I3B. Similarly a portion of the output of the linear amplifier 14G is taken off through a condenser 2| and a rectifier 22 to modify the energy in the blue channel at either or both of the linear amplifying stages I2B and MB. The potentials on the points 23, 24, 25, and 26 depend respectively on the output of the condensers 2D and 2| and the setting of the variable resistances l1, l6, l9, and i8. If, as in the example discussed in connection with Fig. 1, the non-linear amplifiers I3G and [3B have transmission functions respectively according to the square root and third root powers, this arrangement shown in Fig, 2 is completely adjustable whereby the output of the blue circuit may be modified by the energy of the green circuit in accordance with any additive combination of linear, square root, three-halves and third powers. More correctly the output of the blue channel will be in accordance with the third root power of the input of the blue channel modified by a function of the energy in the green channel which-includes additively, a linear function, a square root function, a cube function, i. e. a third power function and a three-halves power function. If the tubes are operating on a straight line portion of their characteristic curves, this modification will be multiplication as discussed above. Otherwise almost any type of modifying is possible by suitably adjusting the tube responses.

The rectifiers Z2 and 22' act merely as rectifiers and have no appreciable effect on the nonlinearity which is established in the non-linear amplifiers [3B and I3G. The varistors 29 in these units are shown as rectifiers, but they should not be confused with 22 and 22' Whose action is actually to rectify.

Fig. 3 shows a preferred embodiment of the invention wherein through suitable modifiers 30, 3|, 32, and 36 Various modification functions are introduced. The light valve [5R of the red channel varies according to some power function introduced by the non-linear amplifier ISR, of the output of the photoelectric cell HR. The light valve IEG in the green channel varies linearly with the output of the photoelectric cell H G or if a non-linear function thereof is desired, the

amplifier 35G by which the light valve lG is operated may include a non-linear stage; The output of the photoelectric, cell 1 IG, is however modified linearly by the modifier in accordance with the output of the photoelectric cell HR and non-linearly by the modifier 32 in accordance with the output of the non-linear'amplifier HR. The light valve [5B in the blue circuit operates non-linearly in accordance with the output of the photoelectric cell IIB passing through the non-linear amplifier [33. The energy in this channel is modified according to the inverse power of the non-linear amplifier I3B by a modifier 3| which introduces the linear function of the output of the photoelectric cell HR, to the final stage of linear amplification [4B in the blue channel. This energy is also modified nonlinearly in accordance with a non-linear amplifier 33G which operates on a portion of the output of the linear amplifier lZG in the, green channel and introduces its non-linear modifica- 4 tion through amplifier 34G and modifier 36 into the linear amplifier I2B.

By dividing the output from linear amplifier I 2G it is possible to have thelight valve IEG operate linearly therewith or according to some non-linear function which is different from the non-linear function used in the modification of the energy in the blue channel.

Numerous other variations of thisidea will be apparent from this description, but I have found that a non-linear amplifier having a transmission function according to the .40 power when placed in each channel together with a modification of the green channel in accordance with the .45 power of the energy in the red channel and a modification of the energy in the blue channel in accordance with the .40 power of the energy in the green channel, gives satisfactory results. This, of course, reduces the system to a relatively simple form.

For the convenience of those accustomed to think in terms of the masking method'of color correction, the following analogy is given. If the above modification is a direct division, this means that the green intensity is divided-by the .45 power of the red intensity and that the blue'intensity is divided by the .40 power of the green intensity. If the signal intensities were then used directly to control the exposures, this would be the same for example as masking the blue negative 'by a positive of the green negative which positive is developed to a gamma of .40. However all of these signal intensities are also reduced to the .40 power before controlling their respective light valves (which are assumed here to be linear but which may have any response function). Therefore the total. contrast of the printing light is reduced by this power which compensates for the high contrast due to the processing of certain types of monopack films.

For example the blue printing light is proportional to the .40 power of the original blue scanning light minus (.40 .40) power (i. e. theQlG power) of the original green scanning light. The gamma to which the yellow layer is developed or in pho'tomechanical processes, the gamma to which the yellow printer is developed and prints, is about 2.5 so that there is correct tone reproduction in the final reproduction.

The more complicated cases where the above modification is not a direct division and/or the valve response is not linear are almost infinite in number and are increasingly more difiicult to explain mathematically. However it is'obvious that something similar or analogous to this reduction in contrast and to this masking with a correcting mask of lower contrast takes place in various types of modification;

For example with multiplication (as in Fig. 2.) the masking instead of being by. a .40 contrast mask which is a direct positive of the green negative,'is by a .40 contrast mask which is a slightly complicated positive function of the green negative. Thus in every case there is something analogous to the reduction in contrast in ordinary masking.

Referring to the specific form. shown in Fig. 2, the non-linear amplifier I 3G for example may include a resistance in series with one side of the circuit and a shunt across the circuit including an adjustable resistance 28 and two inversely arranged rectifiers 29 parallel to each other and in series with the adjustable resistance 28. By using a resistance of 100 ohms for the resistance 2'! and impedance equal to 2000 ohms in the output of this non-linear amplifier, i. e. the primary coil of the transformer between the nonlinear amplifier IBG and the linear amplifier MG, and by'usin'g rectifiers 29 whose resistance varies from 50f an ohm to 100 ohms depending on the current flowing therethrough, which in turn depends upon the setting of the adjustable resistance 28 and the intensity of the input, it is possible to provide an output which varies .proportionally to a power function of the input, which power is between .25 and 1. This power function must be approximately constant over.

the range of tones to be reproduced. Thus, the non-linear amplifiers described above constitute means for uniformly varying the energies of the signals in accordance with an exponential function, the uniformity being maintained over a wide range. As is well known .tophotographers, most color pictures have a range of intensities of a 1000:1 or better, and hence a range control of less than 15m 1 would be useless and even one less than 30 to 1 would serveonlyfor narrow range pictures such as those entirely in a high key or entirely in a low key.

Having thus described myinvention, I wish to point out that it is not limited to the specific structure shown but is of the scope of the appended claims.

What I claim anddesire to secure by Letters Patent of the United States is: I

1. An electro-optical system for the reproduction of a multi-colored original comprising means for scanning the original in a plurality of colors, photoelectric devices adapted to receive the separate colors from the original for establish ing electric energies corresponding thereto, electric amplifying circuits connected to the outputs of the photoelectric devices, at least one of the circuits including in series an amplifier having predetermined non-linear characteristics, means having input and output sides for modifying one of said energies in accordance with at least one other of the energies, one side of said modifying means being connected to the output of the non-linear amplifier and the other side of said modifying means being'connected to another of the'amp'lifying circuits and devices for controlling light'intensity connected to the output of said circuits, whereby the responses of the latter devices are modifications of the intensities of said separate colors as determined by the characteristics of the non-linear amplifier.

2. An electro-optical system according to claim 1 in which the side of the modifying means connected to the output of the non-linear amplifier is the input side of said modifying means.

3. An ele'ctro-optical system according to claim 1 in which the output side of said modifying means is connected to the output of the nonlinear amplifier.

4. An electro-optical system for the reproduction of a multi-colored original having a plurality of channels respectively carrying energies corresponding to color components of the original and. each including a linear amplifying circuit connected to the output of a photoelectric device, at least one of the channels having a nonlinear amplifying circuit wi-th predetermined nonlinear characteristics connected to the output of its linear amplifying circuit and means connected to the output of the non-linear amplifying circuit for modifying the amplification factor of the linear amplifying circuit in at least one of the other channels, in a manner determined by the characteristics of the non-linear amplifier.

5. An electro-optical system according to claim 4 in which the non-liner amplifying circuit has a transmission function equal to about the .40 power for controlling its output in proportion to the .40 power of its input.

6. An electro-optical system for the transmission and reproduction of a multi-colored original comprising means for scanning the original in a plurality of colors, an electric channel corresponding to each color and including a photoelectric cell adapted to receive one of the colors from the original for establishing in the channel an electric energy corresponding to the intensity of this color, means for modifying the energy in at least one of the channels in accordance with the energy in at least one other of these channels, an amplifier having predetermined non-linear characteristics in series with at least one of the channels for modifying all values of the energy therein and light valves connected to the outputs of at least three of the channels, whereby the responses of the light valves are modifications of the intensities of said separate colors as determined by the characteristics of the non-linear amplifier.

'7. An electro-optical system for the reproduction of a multi-colored original comp-rising means for establishing in separate electric channels, electric energies corresponding to the color components of each point of the original in scanning succession, each channel corresponding to one color, at least one of the channels including a non-linear amplifier having means for varying the energy therein in accordance with an exponential function substantially uniformly over a wide range of energies, means connected between the output of the non-linear amplifier and another of the channels for modifying one of said energies in accordance with at least one other of the energies, and devices for controlling light intensity connected to the outputs of said channels whereby the responses of the latter devices are in accordance with the output of said energy establishing means as modified in a manner determined by the characteristics of the non-linear amplifier.

8. An electro-optical system for the reproduction of a multi-colored original comprising means for establishing in separate electric channels, electric energies corresponding to the color components of each point of the original in scanning succession, each channel corresponding to one color, at least one of the channels including a non-linear amplifier which includes a varistor producing an energy variation in accordance with an exponential function which is substantially uniform over a wide range of energies, means having input and output sides for modifying one of said energies in accordance with at least one other of the energies, one side of said modifying means being connected to the output of the non-linear amplifier and the other side of said modifying means being connected to another of the channels and devices for controlling light intensity connected to the outputs of said chan nels.

9. An electro-optical system for the reproduction of a multi-colored original having a plurality of channels respectively carrying energies corresponding to color components of the original and each including a linear amplifying circuit connected to the output of a photoelectric device, at least one of the channels having a non-linear amplifying circuit connected to the output of its linear amplifying circuit and means connected to the output of the non-linear amplifying circuit for modifying the amplification factor of the linear amplifying circuit in at least one of the other channels, said non-linear amplifying circuit including means for producing an energy variation in accordance with an exponential function which is substantially uniform over a Wide range of energies.

10. An electro-optical system for the reproduction of a multi-colored original comprising means for establishing in separate electric channels, electric energies corresponding to the color components of each point of the original in scanning succession, each channel corresponding to one color, at least one of the channels including a non-linear amplifier having means for varying the energy therein in accordance with an exponential function substantially uniformly over a wide range of energies, light valves for controlling light intensity connected to the output of each of the'channels, and modifying means connected to and operated by the output of said non-linear amplifier for modifying the response of the light valve inat least one other of the channels in accordance with the energy in the one of the channels including the non-linear amplifier.

11. An electro-optical system for the reproduction of a multi-colored original comprising means for scanning the original in a plurality of colors, photoelectric devices adapted to receive the separate colors from the original for establishing electric energies corresponding thereto, electric amplifying circuits connected to the outputs of the photoelectric devices, at least one of these circuits including a non-linear amplifier having means for varying the energy therein in accordance with an exponential function substantially uniformly over a Wide range of energies, light valves for controlling light intensity connected to the output of each of the circuits, and modifying means connected to and operated by the output of said non-linear amplifier for modifying the response of the light valve in at least one other of the circuits in accordance with the energy in the one of the circuits including the non-linear amplifier.

VINCENT C. HALL. 

